An ERF is a fluid the rheology characteristics of which vary reversibly and rapidly up to a remarkable extent, in response to application of an electric field.
For example, an ERF of particle dispersion system, which comprises an insulating oil suspended with solid particles, has such a characteristic that, upon application of an electric field, the ERF transforms from a Newtonian fluid into a Bingham fluid which exhibits a yield value corresponding to a magnitude of the applied electric field.
There have been made a variety of proposals such as a variable dumper and engine mount, as those machine control systems which utilize the unique characteristic noted above.
For example, servo valve and hydraulic actuator both adopting the ERF as working oil can be drastically downsized since they are free from movable parts, and it is expected to apply them such as to those fields of a fine operation and micromechanics.
Further, those actuators adopting a system, in which a force is transmitted and output from a driving part via ERF to a driven part, can realize a precise force control with high-speed response, so that such actuators have been applied to a prototype of a force sense presenting device in a virtual reality system.
Unfortunately, most of such various approaches have rarely succeeded in actually reaching a practical level.
One reason therefor will be that the aforementioned control systems have failed to pay attention to a Bingham fluid characteristic which is the most remarkable feature the ERF has. As such, the best use of uniqueness of ERF as a control device has not been sufficiently realized.
There will be now explained the aforementioned Bingham fluid characteristic, hereinafter.
Generally, it is possible to represent a shearing stress .tau. of a Bingham fluid, by a sum of: a yield shearing stress .tau..sub.s to be maintained even after the flowing has begun; and a shearing stress .tau..sub.v due to viscosity; by the following equation: EQU .tau.=.tau..sub.s +.tau..sub.v =.tau..sub.s +.eta..sub.B.multidot..gamma. (1)
wherein .gamma. is a shear rate, and .eta..sub.B is a viscosity coefficient which is linear to the shear rate of the Bingham fluid.
The variable Bingham fluids have such a property that their yield shearing stress .tau..sub.s scan be variably controlled.
Among them, an ERF has such a property that its yield shearing stress .tau..sub.s can be variably controlled by an applied electric field. The fluid characteristics of ERF can be represented by the following equation: EQU .tau.=.tau..sub.E (E)+.eta..multidot..gamma. (2)
wherein .eta. is a viscosity coefficient at the time of no electric field, E is an applied electric field, and .tau..sub.E (E) is a yield shearing stress induced by application of the electric field (.tau..sub.E (E) shall be referred to as "induced yield stress" hereinafter).
As a typical example, an ERF, which comprises a silicone oil and carbon particles, surfaces of which are applied with insulation treatment, has a property to exhibit a substantially constant shearing stress irrespectively of the value of the shear rate, upon application of an electric field. Namely, the viscosity has an effect remarkably smaller than that of the induced yield stress.
Thus, it can be said that an ERF generates a finite shearing stress corresponding to a magnitude of an applied electric field, even when a shear rate is extremely small.
Then, by persistently interpreting the change of shearing stress represented by the equation (2) as being an apparent change of viscosity relative to a shear rate, the shearing stress can be represented by the following equation: EQU .tau.=.eta.(E).multidot..gamma.=(.eta.+.tau..sub.E (E)/.gamma.).multidot..gamma. (3)
wherein .eta.(E) is an apparent variable viscosity coefficient.
As noted above, since ERF generates a finite shearing stress even relative to a small shear rate, the following equation can be established: EQU lim.sub..gamma..fwdarw.0.eta.(E)=lim.sub..gamma..fwdarw.0 [.tau..sub.E (E)/.gamma.]=.infin. (4).
Namely, an ERF applied with an electric field can be regarded as a fluid having an infinite viscosity coefficient under the condition of .gamma.=0. This is the Bingham fluid characteristic just noticed in the present invention.
There has been reported an example which has noticed this Bingham fluid characteristic, in an article titled "Experimental Evaluation of Precision Positioning Mechanism Using Bingham Fluidity of Electro-Rheological Fluid" as presented in an Autumn Lecture Meeting for Oil/Air Pressure held Oct. 23, 1997. However, the article has merely mentioned a basic concept, without referring a concrete development for a practical use.
In view of the above, it is therefore an object of the present invention to fully utilize the characteristics of variable Bingham fluids (inclusive of those mediums having characteristics same with those of the Bingham fluid characteristics), to thereby enhance a practical applicability of variable Bingham fluids to a machine control system.